In recent years, a radio communication apparatus such as a smartphone may include a plurality of antennas for the purpose of improvement of a throughput thereof. That is, for example, Multi Input Multi Output (MIMO) communication is executed by utilizing a plurality of antennas, and thereby, a throughput of a radio communication apparatus can be improved. In particular, in a case where a large amount of downlink traffic from a base station apparatus toward a radio communication apparatus is provided, it is effective to introduce MIMO communication into downlink communication.
In a case where MIMO communication is introduced into downlink communication, a radio communication apparatus includes at least one main antenna and at least one sub-antenna. The main antenna is used at a time when an uplink signal from a radio communication apparatus toward a base station apparatus is transmitted, while the main antenna and the sub-antenna are used at a time when a downlink signal is received in the MIMO communication.
Thus, the main antenna is also used for transmission of a signal, and hence, a characteristic of the main antenna is generally emphasized at a time of design of the radio communication apparatus. That is, arrangement or the like of the main antenna and the sub-antenna is determined in such a manner that, for example, a Signal to Noise Ratio (SNR) of the main antenna satisfies a predetermined criterion. For this reason, a characteristic of the sub-antenna may be inferior to a characteristic of the main antenna.
Hence, in order to improve a characteristic of the sub-antenna and improve a throughput thereof, a Low Noise Amplifier (LNA) may be arranged in a receiving circuit that is connected to the sub-antenna. An LNA is an amplifier with a small Noise Figure (NF) and can amplify a signal received by the sub-antenna without adding a high level of noise thereto. Such an LNA is arranged in the receiving circuit that is connected to the sub-antenna, and thereby, for example, degradation of a characteristic of the sub-antenna due to antenna arrangement can be compensated for.    Patent document 1: Japanese National Publication of International Patent Application No. 10-510965    Patent document 2: Japanese Laid-open Patent Publication No. 2014-187428
Meanwhile, carrier aggregation that assigns a plurality of carriers with different frequencies to one radio communication apparatus to execute communication has recently been put into practical use for the purpose of further improvement of a throughput thereof. That is, for example, a carrier in the 800 MHz band and a carrier in the 2 GHz band are assigned to one radio communication apparatus and the radio communication apparatus may transmit and receive signals of both carriers.
Specifically, the radio communication apparatus uses a carrier in a frequency band that is one of the 800 MHz band and the 2 GHz band to execute Primary Cell connection (that will be referred to as “P cell connection” below) to a base station apparatus and uses a carrier in the other frequency band to execute Secondary Cell connection to the base station apparatus. In a case where carrier aggregation is introduced into downlink communication, the radio communication apparatus transmits a signal of a carrier that is used for P cell connection from a main antenna while receiving signals of the carriers in both frequency bands from the main antenna and the sub-antenna.
Thus, signals that are received by the sub-antenna include signals with different frequencies, and hence, the received signals are band-limited by, for example, bandpass filters and subsequently input to the LNAs as described above. That is, signals in respective carrier bands are individually amplified by the LNAs in the receiving circuit that is connected to the sub-antenna.
However, at a time of carrier aggregation, there is a problem in that no throughput can be improved sufficiently even when the LNAs are arranged in the receiving circuit. Specifically, as described above, bandpass filters or the like are arranged in front of the LNAs, in order to obtain signals in respective carrier bands. For this reason, a cumulative NF in a circuit where the received signals are input to the LNAs is increased, and there is a certain limitation to improvement of a characteristic of the sub-antenna even when the received signals are amplified by the LNAs. As a result, reception quality of the sub-antenna does not satisfy a predetermined criterion and causes its retransmission or the like, so that no throughput may be improved sufficiently.